Answer: A
The unary plus operator tries to convert an operand to a number. true is 1, and false is 0.
The string 'Lydia' is a truthy value.
By using the not operator at the beginning of truthy value it became false.

Answer: A
In JavaScript, all object keys are strings (unless it's a Symbol). Even though we might not type them as strings, they are always converted into strings under the hood.
JavaScript interprets (or unboxes) statements. When we use bracket notation, it sees the first opening bracket [ and keeps going until it finds the closing bracket ]. Only then, it will evaluate the statement.
mouse[bird.size]: First it evaluates bird.size, which is "small". mouse["small"] returns true
However, with dot notation, this doesn't happen. mouse does not have a key called bird, which means that mouse.bird is undefined. Then, we ask for the size using dot notation: mouse.bird.size. Since mouse.bird is undefined, we're actually asking undefined.size. This isn't valid, and will throw an error similar to Cannot read property "size" of undefined.

Answer: A
In JavaScript, all objects interact by reference when setting them equal to each other.
First, variable c holds a value to an object. Later, we assign d with the same reference that c has to the object.
When you change one object, you change all of them.

7. What's the output?
let a = 3;
let b = new Number(3);
let c = 3;
console.log(a == b); console.log(a === b); console.log(b === c);

A: true false true
B: false false true
C: true false false
D: false true true

Answer: C
new Number() is a built-in function constructor. Although it looks like a number, it's not really a number: it has a bunch of extra features and is an object.
When we use the == operator, it only checks whether it has the same value. They both have the value of 3, so it returns true.
However, when we use the === operator, both value and type should be the same. It's not: new Number() is not a number, it's an object. Both return false.

8. What's the output?

class Chameleon {
static colorChange(newColor) {
this.newColor = newColor;
return this.newColor;
}

constructor({ newColor = 'green' } = {}) {
this.newColor = newColor;
}
}

const freddie = new Chameleon({ newColor: 'purple' });
console.log(freddie.colorChange('orange'));

A: orange
B: purple
C: green
D: TypeError

Answer: D
The colorChange function is static. Static methods are designed to live only on the constructor in which they are created, and cannot be passed down to any children or called upon class instances. Since freddie is an instance of class Chameleon, the function cannot be called upon it. A TypeError is thrown.

https://thedailyfrontend.com/what-is-iife-immediately-invoked-function-expression-in-javascript/

Let's understand What is IIFE (Immediately Invoked Function Expression) in Javascript with examples & usecase

Answer: A
This is possible in JavaScript, because functions are objects! (Everything besides primitive types are objects)
A function is a special type of object. The code you write yourself isn't the actual function. The function is an object with properties. This property is invocable.